1. Field of the Invention
This invention is in the field of heat treating materials for the calcination of cement and involves preheating the material in a heat exchanger, and then partially pre-calcining the material as it leaves the hottest stage of the multi-stage heat exchanger in separate stages, in one of which it is combined with rapidly oxidizing fuel components, and in the second of which it is treated with more slowly oxidizing fuel components which are carried over into the final calcination stage.
2. Description of the Prior Art
In the production of cement, it has been suggested that an additional calcination device be included in the gas conduit between a sintering furnace consisting of a rotary kiln and the associated heat exchanger system in order to carry out the deacidification of the raw material almost completely in the heat exchanger system. The rotary kiln is employed almost exclusively for the formation of clinker, i.e., burning of the material to completion. It is also known that the deacidification of a particle of raw material in suspension is dependent essentially upon the temperature, the duration of treatment, and the granulation or range of grain sizes.
Attempts to control the temperature have been made which involve adding fuel in finely divided form to the raw meal in the calcination device preceding the rotary kiln. In this way, a relatively uniform calcination of the raw material can be obtained. A disadvantage, however, was found in this connection that the fuel burns so rapidly that the raw meal particles are subjected to high temperatures in shock treatments. A deacidification of the particles of the meal is achieved to a certain degree, but because of the short period application of high temperatures, the cores of the particles of raw meal are not calcined. In addition, the formation of molten phases and deposit problems in the heat exchanger could not be prevented.
In the aforementioned co-pending application U.S. Ser. No. 932,902, there is described a means for controlling the deacidification of the calcium carbonate portion of the raw material for cement. The duration of treatment of the raw meal particles at high temperature is controlled by utilizing rapidly oxidizing fuel components in a first process stage of a separate calcination process, whereby the rapidly oxidizing fuel components give off their heat content completely, particularly to the finely divided portion of the raw meal. Slowly oxidizing fuel components are added to a separate calcination process in a second process stage, so that before the sintering occurs there is a further deacidification, particularly of the coarse portion of the raw meal. Because of the separate calcination processes there results a particularly long duration time for the material to be deacidified at a relatively high temperature level. With the process described in the aforementioned pending application, the temperature and duration time are so controlled that the deacidification of a particle of raw meal may be controlled and optimized to a substantial extent.